1. Field of the Invention
This invention relates to a catalyst for reducing nitrogen oxides, and more particularly to reducing nitrogen oxides in oxygen-rich exhaust streams, such as the exhaust of lean-operating engines.
In the operation of internal combustion engines, an inherent fuel economy is achieved through the use of lean air/fuel mixtures. However, the lean operation of internal combustion gasoline engines and the normal operation of diesel engines results in the generation of significant quantities of nitrogen oxides, generally signified as NO.sub.x, which have been recognized as environmental pollutants. Efforts towards the abatement of the release of NO.sub.x into the atmosphere include the treatment of NO.sub.x -containing exhaust gases by exposure to catalysts effective to convert NO.sub.x to less noxious species, e.g., nitrogen and oxygen. While some catalysts, commonly known as "three-way" catalysts, are capable of converting NO.sub.x to nitrogen and oxygen while simultaneously oxidizing any unburned hydrocarbons or carbon monoxide in the waste gas stream, such catalysts are only effective for the reduction of NO.sub.x with exhaust gases produced with near-stoichiometric air/fuel mixtures. Accordingly, if the engine is run under lean conditions to take advantage of the fuel economies mentioned above, a three-way catalyst would not be suitable to convert NO.sub.x in the waste gas. Furthermore, three-way catalysts are not effective for NO.sub.x abatement in connection with diesel exhaust, since diesel engines operate almost entirely under lean conditions.
A catalytic material known to be effective for the abatement of NO.sub.x produced under lean conditions comprises a zeolite material that has been ion-exchanged with copper. However, such zeolite catalysts typically exhibit inadequate catalytic activity at normal exhaust gas temperatures and are of limited durability in such service. Accordingly, there is a need for a NO.sub.x abatement catalyst that operates at a lower temperature and that exhibits greater durability than catalysts of the prior art.